Control valves are used in process control systems to manipulate a flowing fluid, to regulate a process variable to a desired set point, to convey or deliver a fluid from a source to a destination, etc. A control valve assembly typically includes a valve body, a shaft or stem, and an actuator to provide the motive power via the shaft or stem to operate the valve or position a plug or flow control member within the valve. A common type of actuator for use with a control valve assembly is a spring and diaphragm pneumatic actuator, which is commonly referred to as diaphragm actuator.
Typically, diaphragm actuators have a casing to house a diaphragm, a diaphragm plate, an actuator stem, and a spring assembly having one or more springs. The spring assembly applies a force against the diaphragm plate to return the actuator stem and a valve or other operator coupled to the stem to a known position in the absence of a control pressure applied to the diaphragm. In the case where a single spring is used to implement the spring assembly, the spring is typically centrally located on the diaphragm plate. Where multiple springs are used, the springs are typically distributed circumferentially about the center of the diaphragm plate and apply their respective forces directly to the diaphragm plate.
Regardless of whether one or multiple springs are used to implement the spring assembly, diaphragm actuators receive a control pressure to vary gas (e.g., air) pressure on one side of the diaphragm to move or stroke the actuator stem and thereby open and close or modulate a control valve that may be coupled to the actuator stem. The amount of control pressure required to move and maintain the actuator stem and, thus, the valve or other operator controlled by the actuator, at a given position along its range of stroke, typically equals the force exerted by the spring assembly plus the force exerted by the valve or other operator on the actuator stem. The force exerted by the spring assembly typically increases in a substantially proportional and linear manner as the actuator stem moves toward its fully stroked condition.
Additionally, the force exerted against the actuator stem by the valve or other operator may also vary (e.g., increase or decrease) as the position of the valve (e.g., the position of a valve plug, disc, etc.) changes from its zero stroke to its fully stroked condition. Such changes in the force exerted by the valve stem may be due to the flow characteristics of the valve, the pressure and flow rate of the fluid being controlled by the valve, etc. and, thus, may be substantially nonlinear. Thus, the amount of control pressure required to fully stroke the actuator and the valve or other operator coupled to the actuator may be relatively high and, as a result, may require the diaphragm, the casing, and other actuator components to withstand relatively high pressures. Further, because the force exerted by the actuator spring assembly typically varies linearly over the range of the actuator stroke and because the force exerted by the valve on the actuator stem may vary non-linearly or in some other manner, the position of the valve may vary in an undesirable manner relative to control pressure (e.g., substantially non-linearly).